High frequency electron discharge devices having improved mode suppression means for cavities with re-entrant drift tubes



March 24,1970 F. l. FRIEDLANDER ET AL 3,502,934

7 CES HAVING IMPROVED HIGH FREQUENCY ELECTRON DISCHARGE DEVI MODE SUPPRESSION MEANS FOR CAVITIES WITH RE-ENTRANT DRIFT TUBES Filed Sept. 15, 1967 2 Sheets-Sheet 1 Fl .l v/.

INVENTORS FRED LFRIEDLANDEH PETER J. SPALLAS &m W

ATTORNEY United States Patent US. Cl. 315-5.39 8 Claims ABSTRACT OF THE DISCLOSURE Undesired TE type modes in re-entrant cavity resonators of high frequency electron discharge devices which include klystron and coupled cavity slow-wave circuits incorporating re-entrant drift tube sections can be suppressed to a great extent by a combination of lossy loading material deposited on the surfaces of axially slotted drift tubes in the region of the slots. The axial slots provide a dual function of reducing the resonant frequency of TE type modes in re-entrant cavitie thus permitting the tube designer to eliminate any correspondence between the resonant frequency of undesired TE type cavity modes and second harmonics of the operating mode by tuning the mode either above or below the 2nd harmonic as well as increasing the current path lengths of the undesired TE type modes to permit easy reduction of the unloaded Q (Q of the mode without substantial perturbation of operating mode of the cavity and device. The utilization of lossy R.F. attenuating means in the slotted regions provides excellent stabilization by further reduction to Q for the undesired modes.

In United States patent application Ser. No. 342,217 by Albert D. La Rue et al., filed Feb. 3, 1964, and assigned to the same assignee as the present invention a rather thorough discussion of the nature of cavity oscillations and a novel solution for suppression of same is given which involves the utilization of distributed loss techniques.

BRIEF SUMMARY OF THE INVENTION This invention relates in general to the field of high frequency electron discharge devices operable in the microwave spectrum and more particularly to improved mode suppression means for undesired modes in re-entrant cavity resonator portions of such devices.

The excitation of undesired cavity modes in a high frequency electron discharge device is deleterious to the device operating efiiciency as well as to the quality of the output signal and can produce undesired spurious signals as well. As operating powers increase beam currents increase and the chances of reaching the start oscillation current value for any given undesired cavity mode increase accordingly. This invention is concerned with providing a novel mode suppression approach for high frequency electron discharge devices incorporating re-entrant cavity resonators which has been found to be particularly useful in reducing the Q for TE types of modes and particularly for the TE type of mode. The invention in its basic form involves the introduction of a plurality of elongated slots in the re-entrant drift tube portions which has been determined to provide increased current path lengths for the TE type of mode without substantially perturbing the operating mode. The length of the slots which form a plurality of fingers can be used to control the resonant frequency of the undesired mode to a considerable degree as Well as the reduction in Q for the undesired mode and by incorporating lossy R.F.

attenuating material on the drift tube slotted surfaces an additional significant reduction in Q for the undesired mode is achieved. Good results have been obtained by using lossy material on the inside, the outside and both inside and outside of the re-entrant drift tubes in conjunction with radial and nonradial slots as well as with in-phase and out-of-phase slots.

It is therefore an object of the present invention to provide improved mode suppression means for high frequency electron discharge devices operable in the microwave spectrum and incorporating re-entrant cavity resonators.

A feature of the present invention is the provision of a high frequency electron discharge device incorporating cavity resonators with re-entrant drift tubes with a plurality of slots in the re-entrant drift tubes.

Another feature of the present invention is the provision of a high frequency electron discharge device incorporating cavity resonators with re-entrant drift tubes with a plurality of slots in the re-entrant drift tubes in conjunction with lossy R.F. attenuating material in the slotted regions.

These and other features and advantages of the present invention will become more apparent upon a perusal of the following specification taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic representation of a high frequency electron discharge device incorporating the teachings of the present invention,

FIGS. 2A and 2B are enlarged transverse and longitudinal cross sectional views of the s ructure of FIG. 1 with the transverse view taken along lines 2-2 of FIG. 1 in the direction of the arrows,

FIGS. 2C-2H are views similar to that of FIG. 2A depicting alternative embodiments of the present invention,

FIG. 3 is a cut-away view of a re-entrant cavity resonator incorporating an out-of-phase slotted embodiment of the present invention,

FIGS. 4 and 5 are enlarged cross-sectional views taken along the lines 4-4 and 55, respectively of the embodiment depicted in FIG. 3,

FIG. 6 is a graphical plot of the variation of f for TE type of modes with slot depth for various slot arrangements,

FIG. 7 is an illustrative graphical portrayal of the relationship between a typical fundamental and second harmonic frequency spread for S-band,

FIG. 8 is a transverse sectional view of a cavity resonator with re-entrant drift tubes depicting the field configuration for the operating TM type mode,

FIGS. 9A and 9B are longitudinal cross-sectional top and side views of a cavity resonator with re-entrant drift tubes showing the field configuration for a TE type mode,

FIG. 10 is a transverse sectional view of the re-entrant cavity resonator of FIG. 9A showing TE mode pattern as a complete transverse section through the cavity midplane in the area bounded by the drift tubes,

FIG. 11 is a transverse sectional view of a re-entrant cavity resonator depicting a pair of orthogonal 'I'E modes in the area bounded by the drift tubes,

FIG. 12 is an illustrative graphical portrayal of the effects of slotted drift tubes on Q for a TE mode both with and without loading in the slotted regions.

Turning now to FIG. 1 there is depicted a high frequency electron discharge device 10 which includes electron beam forming and projecting means 11 disposed at the upstream end portion of the device, electron beam collector means 12 disposed at the downstream end portion of the device and a plurality of re-entrant cavity resonators 13 disposed along the beam path therebetween.

The electron beam forming and projecting means 11 can be of any conventional design such as, for example, :1 Pierce type gun having cathode emission surface 14, focusing anode 15 and main accelerating anode 16. A thermionic heater 17 can be used to induce thermionic emission from the cathode. Any conventional R.F. coupling means such as coaxial coupler 18 can be used to introduce R.F. energy into the input cavity and any suitable R.F. coupling means such as, e.g., waveguide 19 can be coupled to the output cavity or in a hybrid tube type the downstream portion of the slow wave interaction section to extract amplified R.F. energy.

Since the theory and operation of both the multicavity klystron amplifier of types such as depicted for example in US. Patent No. 3,169,206, by R. B. Nelson, issued Feb. 9, 1965, and assigned to the same assignee as the present invention, and hybrid devices which incorporate klystron buncher sections and traveling wave output sections such as, for example, shown in US. Patent No. 3,289,032 issued Nov. 29, 1966, by R. R. Rubert et al. and assigned to the same assignee as the present invention are well known, further elucidation on the general properties of these devices will not be given herein.

The Q, reduction and frequency shifting teachings of the present invention are applicable to standard klystron type cavity resonators having re-entrant drift tubes such as illustrated in FIG. 1 or in hybrid tubes as discussed above as well as to slow-wave circuit traveling wave tubes incorporating coupled cavities having re-entrant drift tubes. The slow-wave coupled cavity circuits will differ from the klystron cavity circuits by the presence of additional coupling slots or regions between cavities which provide the circuit with a passband characteristic, (see for example, FIG. of US. Patent No. 2,636,948) such that traveling wave tube operation is possible. These traveling wave tube coupled cavity resonators with re-entrant drift tube regions can support undesired TE type modes also and the teachings of the present invention are applicable thereto although specifically illustrated and exemplified with respect to the klystron. Therefore the terminology cavity resonators having re-entrant drift tubes as used hereinafter in the specification and claims shall be applicable to those coupled cavity slow-Wave circuits which also utilize re-entrant drift tubes.

The high frequency electron discharge device depicted in FIG. 1 is representative of both the straight klystron type and the hybrid types which incorporate cavity resonators having re-entrant drift tubes 20.

In order to better understand the contribution of the present invention the following general discussion regarding the importance of relative Qs for cavity resonators is given.

One may take the total effective cavity Q for any mode Q, is the total effective cavity Q Q, is the Q resulting from beam loading alone Q is the normal unloaded cavity Q Q is the Q resulting from any external load alone.

It is plain that Q will be reduced by any reduction of Q or Q... Thus an external load reducing Q or the use of lossy material reducing Q results in a reduction of Q Q the beam loaded Q, may be negative or positive, depending on the gap conductance for the mode in question. If 1/ Q, is negative and larger in magnitude than l/Q +l/Q Q, is negative, and oscillation is possible. One must therefore arrange conditions so that the sum of the positive terms on the right is larger in magnitude than the negative term 1/ Q, (if it should be negative).

In the present case we are concerned with how to effectively reduce Q for TE type modes such that Q, will remain positive for these general type of modes. Experimental analysis of a multicavity klystron amplifier operable in the S-band region of the microwave spectrum at a perveance of 3 1() in an attempt to design improved high power klystrons resulted in oscillations between 8100 mh. and 8175 mh. starting at 30 amps peak current. Extensive experimental analysis finally led to the conclusion that a TE type of mode was responsible for the oscillations. Further analysis of this type of mode led to the finding that mode was characterized in a re-entrant cavity resonator by strong transverse E fields in the vicinity of the interaction ga and internally of the drift tubes and had substantial axial E field components at the edges of the drift tubes.

The application of simple lossy R.F. coating on the eX- terior of the drift tubes as taught in copending US. Patent application Ser. No. 342,217 is not the optimum solution for Q reduction for this mode.

It was discovered that by introducing a plurality of elongated slots which produce a plurality of substantially axially directed and circumferentially spaced current flow paths in the re-entrant portions of the drift tubes such as to form a plurality of fingers that it was possible to both reduce Q for TE type modes and to tune the mode down in frequency which not only provided increased current flow paths but enables the tube designer to shift the resonant frequency of the mode sufficiently to preclude any correspondence between the i of the mode and second harmonics of the fundamental in the operating band of the tube which can be quite wide for heavily loaded cavities and especially so if broad band techniques and tunable cavities are provided. In other words, the mode can be tuned above or below the 2nd harmonic as desired.

A particularly desirable feature of the use of elongated slots in re-entrant drift tubes is the fact that very little effect on the fundamental mode occurs. For example, an S-band cavity with a gap of 0.582 inch and a symmetrical pair of drift tubes each having a re-entrant length of .435 inch was analyzed and found to support a TE mode at around 8100 to 8175 mh. The drift tubes were then slotted with 16 radial slots cut in each drift tube to a depth of .375 inch each. This produced a shift in the f of the TE mode down to around 6000 mh. and a shift of only around 6 mh. for the fundamental TM type mode. Similar results were obtained with diverse types of slotting arrangements which shows the excellent mode selectivity of the slotted drift tube technique. By combining the slotted re-entrant drift tube approach with lossy R.F. attenuating coatings on the drift tubes substantial reductions in Q for the TE type of mode were obtained. Good results were obtained with various types of slotting approaches and attenuating coatings. For example, Q reductions of better than 5 to 1 were obtained by using various combinations of slots and internal, external and both internal and external lossy R.F. coatings on the re-entrant portions of the drift tubes as will be shown in more detail in the accompanying tables set forth hereinafter.

As discussed previously, if positive stability for any given undesired mode is wanted, it is merely necessary to design the cavity such that 1 1 1 1 Qt Qb Qo Qe is positive for a particular mode.

If l/Q is negative and Q5 is smaller than 1 1 Q QB then oscillations can occur. Thus, if one neglects Q entirely it is possible to assure stability by simply making Q for a given undesired mode smaller than Q, for that mode. Any Q will obviously increase the stability safety factor. By combining the slotted drift tube approach with 5 lossy R.F. attenuating material it is very easy to do this since slotting by itself may not provide suflicient reduction in Q for the mode. At any rate the present invention is not restricted to any particular degree of reduction in Q for undesired modes since it is clearly a matter of individual design as to what values of Q Q, and Q, for a given mode under a given set of operating conditions are desired.

Turning now to FIG. 2A an embodiment of the slotted drift tube concept is depicted which utilizes 16 radial slots 21 in conjunction with lossy R.F. attenuating material 22 on the external portion of the drift tube 20* as shown.

Good results were obtained with a Kanthal A applied by standard flame spraying techniques. Kanthal A is an alloy of aluminum, 22% chromium, 0.5% cobalt and the balance iron. Naturally the thicker the coating (up to the skin depth for the particular frequency spectrum concerned) the better the Q reduction for the undesired mode.

Another embodiment of the present invention is depicted in FIG. 2B wherein 16 radial slots are shown in combination with lossy R.F. attenuating coating 22 on the exterior of the drift tube and 23 on the interior of the drift tube. Good results were obtained with flash plated iron deposited from an electrolytic bath in a well known manner to any desired thickness. The lossy R.F. attenuating material can obviously be of any commercially available type that is capable of handling the power levels involved and is not restricted to the above types. In both the embodiments of FIGS. 2A and 2B the slots were cut in opposing drift tubes in-phase or axially aligned.

In FIG. 2C a variation using parallel pairs of nonradial slots is depicted which combined 4 nonradial slots 24 with lossy R.F. attenuating material 25 on the external surface. This variation also produced good results.

In FIG. 2D a variation using nonradial and nonparallel slots 26 in conjunction with loosy R.F. attenuating material 27 is depicted. This approach also produced good results.

In FIGS. 3, 4 and 5 an out-of-phase slotted approach is depicted which was also found to be effective. It combines two pair of parallel nonradial slots 28, 29 in each drift tube which are 90 azimuthally space rotated in opposing drift tube sections in conjunction with lossy RF. attenuating material 30 of the type set forth previously.

In each case if it is desired to minimize the chances of any of the lossy coating from breaking away from the slotted regions it may be removed from the slots or suitable masking may be employed so that only the exterior or interior or both surfaces are coated.

If it is desired to minimize loading of the fundamental mode in certain cavities such as, for example, the output cavity the lossy coating is deposited on the inside or interior of the drift tube only or on the inside and only partially on the outside exterior of the drift tubes. For example, it can be removed from the gap ends.

Now it has also been determined experimentally that the introduction of any asymmetries in the cavities such as asymmetries in the slots or uneven distribution of lossy R.F. attenuating material can produce mode splitting of the TB type of mode which results in two orthogonal field patterns. This mode splitting will result in a distinct spaced pair of resonant frequencies such as denoted by A, C in FIG. 7. See FIG. 11 for a simplified pictorial representation of the E and H field patterns for this split mode.

Now the resonant frequency of the mode least effected by the slots may have risen from the unsplit value, B in FIG. 7, to A in FIG. 7. This means you will not have to tune a nonsplit TE mode down as far as a split mode to get out of the 2nd harmonic region, if the desired Q reduction requires that the mode be tuned down into or below the 2nd harmonic. It is preferable not to have the mode in the second harmonic frequency band, since sec- 0nd harmonic power may then be coupled into the mode and cause cavity heating and/or beam deflection by the transverse fields as well as increased instability. The use of a pair of in-phase nonradial slots such as shown in FIG. 20 has been found to produce a definite TE mode split into an orthogonal pair. This mode splitting can be alleviated however, if desired, by using the out-of-phase nonradial approach of FIGS. 3-5. This will effectively equally tune each of the orthogonal branches and produce a nonsplit TE type mode with a simple f as at B in FIG. 7. In any case, greater design freedom is obtained by preventing mode splitting.

Various combinations of slotting and loading techniques were investigated with the following results:

TABLE I.-SUMMARY OF EXPERIMENTAL RESULTS FOR TE MODE LOADING Slots TEu Mode Depth, Cavity No. in. Rad/Nomad Drlft-Tube Lossy Coating 11,, Mb. Q Af/f,,

4 0 0 N n g l iggll s r kan gialed) E 3, 165 1, 360 2. 34 oug an a on cy in rice ,210 144 2. 17 4 4 In Phase -i part; of outside. 7, 416 86 2.05 433 Out of phase, non- Rough kanthal on cylindrical 6,070 108 1. 74 4 8 radial. part of outside 0.1).). 6, 165 41 1. 68 4 8 285 In phase, radia1 Rough kanthal on outside ex- 7, 156 99 1. 86

cept .050 at tip. 4 8 285 ..do Rough kanthal on cylindrical 7, 060 202 2,01

part of 0.D. 4 3 400 do Iron plated inside from .030 to 6,224 132 1, 22

.400 from tip. 4 16 .400 do Rough kanthal on O.D. except 5,757 1, 66

.138 from tip. 4 16 390 do Iron plated inside from .030 to 5, 898 151 1. 93

.400 from tip.

4 16 375 do No lossy coating on drift-tube.-- 5, 893 890 1. 90 4 16 375 .do Rough kanthal on cylindrical 5, 939 132 1. 93 part of h ery roug out a on cy- 5, 888 91 1. 58 4 16 lindrical part of 0.1). 5,967 88 1, 54 4 16 365 .do No lossy coating on drift-tubes- 5, 934 660 1, 82 36 d {Rough kanthal on cylindrical 5,986 139 1. 95 4 16 5 part ofO.D. 5,992 117 2,00 4 1 365 d {As above case but slots cleaned 5, 956 1 84 6 or kanthal. 5, 973 157 96 6 0 0 Kanthal on cavity and walls 8, 894 318 only; high radiation loss from from drift-tubes. 6 16 292 In phase radial Iron plated inside from .030 to 7, 083 197 2. 31

.430 from tip. 6 16 292 .do As above case but iron plated 7, 053 138 1. s1

' on O.D. so.

"Af/J'D is proportional to mode impedance, measured with a .125 inch quartz rod on the axis giving a frequency shift of AI, Mh. with a resonant frequency of f in G Of all the above solutions the 16 radial and in-phase combination was found to be preferred from an overall standpoint although each combination set forth above has obvious merits as a means for achieving substantial reduction of Q in TE type modes in re-entrant cavity resonators.

A complete klystron was designed with the Q reduction technique used with the following results when using the preferred in-phase, l6 radial Slot approach:

TABLE II.MEASUREMENTS 0F TEm MODE IN DRIVER CAVIIIES OF MODIFIED KLYSTRON Cavity 1 2 3 4 5 6 Orthogonal TE m Modes:

. 583 58? 582 342 Number of slots 16 16 16 16 16 16 Slot width, in {;2 $62 $62 Hi2 H32 H32 Slot length, in. 401 360 360 360 360 292 Drift-tube lossy Very rough kanthal 0.0001 in. to

coating. 0.0005 in. iron plating. Location oflossy Cylindrical part of OT). On O.D. except coating of drift- .030 in. from tubes. tip, on LB.

from 0.30 in. to .430 in. from tip. Location of kan- End walls and side wall End walls only.

that on cavity walls.

Not measured.

The effects on the f of the TE type mode of the number and kind of slotting technique used and on the depth of the slots is presented in FIG. 6 for a typical reentrant cavity resonator having a pair of .435 inch re-entrant drift tubes using inch wide slots.

Curve A, B, and C are for 4 nonradial slots in each drift tube using two pairs of parallel slots spaced .625 inch apart as shown in FIGS. 2 and 35. Curves A and C represent the orthogonal pair for the in-phase case depicted in FIG. 2 and curve B represents the 90 spaced rotated approach of FIGS. 3-5 which shows that mode splitting is obviated by this technique.

Curve D is for the 8 nonradial slot case such as depicted in FIG. 2D.

Curve E is for the 8 radial slot case using equal spacing between slots which is not separately illustrated but which can be depicted in FIG. 2A, for example, by simply eliminating every other slot.

Curve F is for the 16 radial slot case such as shown in FIGS. 2A, 2B using even spacing between slots.

It is seen that the greater the number of slots the greater the corresponding reduction in f for TE type modes and that the greater the depth of the slots the greater the reduction in for the TE type modes.

These results can be generalized by referring to FIGS. 8 and 9. In FIG. 8 a typical TM type of fundamental mode for re-entrant cavity resonators is depicted. In FIG. 9A and 9B a cross section of a re-entrant cavity resonator shows the TE type of mode which has a full wave pattern around the circumference with maximum transverse fields at the gap and also an appreciable axial component at the edges of the gap. Standard perturbation techniques can be used to identify this type of mode which in a pure cylindrical cavity would occur at a much lower frequency. FIG. 10 shows a transverse section of this mode pattern taken along the midplane of the cavity and in the area bounded by the drift tubes.

The utilization of the slotted drift tube with lossy material in the region of the slots as discussed above will effectively reduce the Q for any mOde which has strong gap fields coupled with appreciable circumferential current flow paths on the drift tubes and in general this means TE mode types in contradistinction to TM mode types. The number of slots can obviously be varied from even the uneven numbers as well as spacing between slots without departing from the scope of the present invention. In

FIG. 12 the effects of variation in Q for the TE mode as a function of slot depth both with and without RF. loading on the drift tubes is depicted for purposes of illustrating the rather startling reduction of Q, for TE modes which occurs for simple slotting even without the use of lossy R.F. attenuating material in the slotted regions. The data were run for a cavity having Kanthal on the cavity side walls with a .582 inch gap and four 1 2 inch slots per drift tube and spaced X inch apart. Curve A is for no R.F. loading on drift tubes and Curve B for R.F. loading on drift tubes. It should be noted that although axially parallel slots are preferred non-axially parallel cases although less effective are included in the teachings as mere deviations of the general preferred case.

In summation then, it is seen that the utilization of the slotted drift tube approach of the present invention results in reducing the r esonant frequency for the TE type mode without substantial perturbation of the operating mode. It is thought that, regardless of whether or not lossy R.F. material is applied to the slotted region of the drift tubes that the use of slots which reduce the resonant frequency of TE type mOdes at least 10% or more with respect to the nonslotted resonant frequency of the TE type mode should be very useful and if the cavity walls are coated with RF. lossy material substaintial decreases in the Q of TE type modes occurs due to the perturbation of the mode which produces increased cavity wall currents. However, the combination of slots and R.F. lossy material in the slotted region has been shown to be most effective.

Since many changes could be made in the above con struction and many apparently widely different embodiments could be constructed without departing from the scope thereof it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A high frequency electron discharge device including beam forming and projecting means disposed at an upstream end portion of said device for generating and directing an electron beam along the central beam axis of the device, a plurality of cavity resonators disposed along the beam axis, said cavity resonators having re-entrant drift tube portions disposed about the central beam axis for permitting the electron beam to pass from cavity resonator to cavity resonator along said central beam axis, at least some of said re-entrant drift tubes being provided with elongated slotted regions which introduce a plurality of substantially axially directed and circumferentially spaced current fiow paths in the re-entrant portion of said drift tubes, said slotted regions introducing at least a 10% or more reduction in the resonant frequency of the TE type cavity mode in the respective cavity resonators in which they are utilized with respect to the resonant frequency of the same TE type cavity mode in the absence. of said slotted regions and wherein said drift tubes are provided with lossy R.F. attenuating material in said slotted regions.

2. A high frequency electron discharge device operable in the microwave spectrum comprising electron beam forming and projecting means disposed at an upstream end portion of the device for producing and directing an electron beam along an elongated central beam axis, electron collector means disposed at the downstream end portion of the device for absorbing the spent beam and beam-wave interaction means disposed along the device axis between said upstream and downstream end portions, said beam-Wave interaction means including at least one cavity resonator having a re-entrant drift tube portion disposed within a cavity end wall about said beam axis, said re-entrant drift tube being characterized by having a plurality of elongated slots forming a plurality of fingers in the re-entrant portion thereof which extend fro-m the free end towards said cavity end wall beyond said free end, said plurality of fingers being provided with lossy R.F. attenuating material.

3. The device defined in claim 2 wherein said lossy R.F. attenuating material is disposed on the internal surface of said drift tube.

4. The device defined in claim 2 wherein said lossy R.F. attenuating material is disposed on the external surface of said drift tube.

5. The device defined in claim 2 wherein said lossy Riff. attenuating material is disposed on both the internal and external surfaces of said drift tube.

6. The device defined in claim 2 wherein said cavity resonator includes a pair of re-entrant drift tubes disposed about said beam axis in axial alignment from the respective cavity end walls and wherein said ,slots are axially parallel to and radial with respect to said beam axis.

7 The device defined in claim 2 wherein said cavity resonator includes a pair of re-entrant drift tubes disposed References Cited UNITED STATES PATENTS 2,939,037 5/1960 Jepsen 3155.52

HERMAN K. SAALBACH, Primary Examiner S. CHATMON, JR., Assistant Examiner U.S. Cl. X.R. 315-552, 5.53; 333-83A 

